Hello tjgray,
PIX has only one outside interface at any stage. you can add up DMZ interfaces but not outside. Are the T1s terminating in two different routers or on a single router ? If on a different router, you can connect this router on a DMZ interface and do statics/NAT selectively to flow through the new T1.
Note: You cannot do load balancing on PIX. If you want internet access through both the routers, it is tough , as you cannot add 2 default routes in PIX. you can selectively put some traffic (IPSEC etc) on the second T1 by adding static routes.
Overall, it is tough in your scenario to use both the T1's simultaneously using PIX..